A four or five-digit PIN is to be selected from the eight digits 0,1,2,3,4,5,6,7.

1. How many 4-digit PINs are possible if: a) all 4 digits are the same? e.g. 3-3-3-3 Correct b) no digit can be repeated? Correct c) any digit can be repeated? Correct d) the first 2 digits must be different digits and the last 2 digits must be the same, but none of the first 2 digits can be used in the last 2? Correct
2. Suppose any digit can be repeated, how many 4-digit PINs are possible if each PIN: a) must start with "6"? Incorrect b) must not end with "2"? Correct c) must neither start with "7" nor end with "7"? Correct
3. Suppose no digit can be repeated, how many 5-digit PINs are possible if each PIN: a) must start with "6"? Incorrect b) must not end with "2"? Correct c) must neither start with "7" nor end with "7"? Correct